Rare earth element-containing alloys composed so as to form the RE.sub.2 TM.sub.14 B tetragonal crystal phase have been melt spun under carefully controlled processing to produce useful permanent magnet materials as disclosed in Croat U.S. Pat. Nos. 4,802,931 and 4,851,058. Such melt-spun materials either as quenched or in an overquenched and annealed condition consist essentially and predominantly of the tetragonal crystal, prototype Nd.sub.2 FE.sub.14 B phase. The tetragonal crystal-containing grains are very small, typically less than a few hundred nanometers on the average in grain size, and are surrounded by one or more secondary grain boundary phases which contribute to the permanent magnet characteristics of the composition. This fine grain material is magnetically isotropic, and the melt-spun ribbon fragments can be pulverized to a suitable powder, combined with a suitable binder material and molded into useful bonded isotropic permanent magnets as disclosed in Lee et al U.S. Pat. No. 4,902,361.
Where permanent magnets of higher energy product are desired, it is known that the melt-spun powder material can be hot pressed to form a fully densified permanent magnet body and that, where desired, such fully densified body can be further hot work deformed into a magnetically very strong, anisotropic magnet. These practices are disclosed, for example, in Lee U.S. Pat. Nos. 4,792,367 and 4,844,754.
The fine grain, melt-spun, rare earth element-containing material is initially in the form of ribbon particles or a powder produced by comminution of the ribbon fragments. In order to hot press or otherwise hot work the material, it is necessary that it be heated to a suitable hot working temperature typically in the range of 700.degree. C. to 800.degree. C. As disclosed by Lee, it is prudent to heat the powder in vacuum or suitable inert gas that provides a dry and substantially oxygen-free environment in order to prevent the powder from burning. In attempting to work with such readily oxidizable rare earth element-containing materials, it has been necessary to provide a suitable protective atmosphere in which the rare earth and other constituents are not oxidized and the permanent magnetic properties of the materials are not degraded.
In co-pending application attorney docket number G-6940, assigned to the assignee of this invention, is disclosed a two-step cold pressing-hot pressing process for producing hot pressed rare earth-transition metal-boron (RE-TM-B) magnets in an open-to-the-air press. In the first step of that process, fine grain RE-TM-B material in powder form is compacted at ambient temperature in open-to-the-air presses. The cold pressed compact that is formed has a density of about 5 to 5.5 grams per cm.sup.3, which is about 70 percent of the density of a fully densified body of the same composition. In accordance with such two-step practice, the cold compacts are then suitably hot pressed in an open-to-the-air hot press in which the die cavity is heated and flooded with a dry inert gas such as argon to protect the compact from burning or other oxidation which would degrade the magnetic properties of the product.
The rare earth element-containing powder and cold pressed compact are susceptible to reaction with moisture and with certain chemical species such as the chloride ion. Therefore, in order to prevent chemical reaction of the constituents of the powder or the compact, it has been necessary to take precautions in addition to the use of dry inert gas during the hot pressing operation. For example, in the cold press-hot press practice referred to above, a lubricant is used in the cold pressing operation to facilitate compaction of the powder and removal of the compact from the die without abrasion of the die or the compact and without causing the compact to split apart. In order to accomplish this successful cold pressing, it was determined that a solid lubricant film, such as a film of polytetrafluoroethane or fluorinated ethylene-propylene copolymers (Teflon), should be applied to the die wall. No lubricant should be mixed with the powdered material added to the die for compaction. If the compact was to be ring shaped or the like and require a core piece as part of the press tooling, a film of solid lubricant could also be suitably applied to the surface of such core piece.
Teflon powder is a preferred solid lubricant film. The application of the powder to the die or core surface is accomplished using a suspension of Teflon in a volatile vehicle, preferably a liquid of relatively high density, so as to better suspend the Teflon powder particles. Volatile chlorine-fluorine containing aliphatic hydrocarbon liquids have been used in the above-described process to suspend the Teflon particles. However, some liquid remains in the lubricant film after drying and is transferred to the cold compact. In some operations, it is necessary or desirable to store cold compacts for hours or days before they are hot pressed. During such time, trace amounts of chlorine-containing liquids or other reactive liquids, particularly humid atmospheres, can react with the rare earth element-containing powder. Such reaction degrades the permanent magnet properties of the resultant product such a by reducing its coercivity.
Accordingly, it is an object of the present invention to provide a suitable liquid vehicle for suspending solid lubricant materials such as Teflon powder for die lubrication in the making of rare earth element-containing cold compacts for subsequent hot pressing.